The oldest type of dental varnishes comprised solutions of copal resin, a fossilized plant-derived mined material, in chlorinated hydrocarbons and later in alcohols. These varnishes were almost exclusively used as coatings over the cavity walls prior to placement of amalgam-type restoratives. They were not suitable for use under the novel resin based restoratives because copal resin acts as an inhibitor of the polymerization process, negatively influencing the cure of the restorative material and, in consequence, the quality of the restoration. Incomplete cure of the material could also cause postoperative discomfort and, in case of vital teeth, serious damage to the pulp. Copal varnishes, although generally beneficial in preventing marginal leakage of new restorations, did not offer benefits of tooth fluoridation and, therefore, did not provide an important, additional measure of protection against secondary dental tissue decays.
Copal resin based varnishes were followed by varnishes based primarily on polyamide-type polymers, representing reaction products of aliphatic diamines with long chain carboxylic acids. Examples of such varnishes are described in U.S. Pat. No. 4,396,378. Synthetic varnishes generally do not inhibit the polymerization process of resin based restorative materials and, therefore, were suitable for use under amalgam as well as under polymeric restoratives. Because of their light consistency and low viscosity, they were unable to maintain fluoride salts in suspension. Their role was, therefore, limited to sealing dentin tubulae without providing the benefits of fluoridizing potentially vulnerable tooth surfaces.
Another type of dental varnish encompasses formulations comprising, in addition to film formers (natural or synthetic resin), therapeutic agents, to effect desirable changes in the chemistry of teeth, reduce tooth sensitivity and to protect vulnerable oral tissues against potentially damaging side effects resulting from contact with dental materials.
The most common among therapeutic additives in modern dental varnishes are fluoride salts, particularly sodium fluoride. The most commonly known fluoride containing dental varnishes are those comprising colophony resins as a film former, ethyl alcohol as a medium/solvent and sodium fluoride as the main, or only, therapeutic agent. Colophony is a natural resin derived from living trees and, as such, its characteristics frequently vary from one lot of resin to another. The differences often include properties relevant for the performance or esthetics of the varnish made with such resins. A desirable consistency of the varnish is important to achieve proper film thickness of the coating, to facilitate handling and, most of all, to prevent sedimentation of fluoride salts and other dispersed components of the formulation. Other disadvantages of these varnishes include their slow solvent release, resulting in a slow cure, and their unpleasant taste.